Thanks a lot for your feedback, it is very much appreciated. It is clear Phattaya is not a province, but it was not clear to me what it really is. Some sources say it has special rights similar to Bankok.
What do you suggest we do with it? How shall we model it?

The admin subdivisions for Thailand are unfortunatley very complex and there are very few good datasources. Your website is by far the best.
What is the license of your datasets? Are we allowed to use it and sync geonames with your data?

Phattaya is a special administrative area, the same term as used for Bangkok. However, both are not the same in their administrative function, as Bangkok is both a province and a local administration, whereas Phattaya is only a local administration. The main root to the confusion is the fact that in Thailand there are two administrative hierarchies in parallel - the central administration changwat, amphoe, tambon, muban - and the local administration with thesaban and TAO, with the local administration often (but not always) having the same boundaries as a corresponding tambon.

Thus Phattaya is still part of Chanburi province and Bang Lamung district, but unlike the other municipalities it is not under supervision of the provincial governor. I don't know why ISO chose to give Phattaya a code like a province, maybe due to this higher degree of self-government.